1. Field of the Invention
The present invention relates to a planographic printing plate precursor for preparing a waterless planographic printing plate and to a production method thereof and, in particular, to a waterless planographic printing plate precursor on which heat mode recording using laser light is possible, and which has excellent scratch resistance, solvent resistance, and image reproducibility and to a production method thereof.
2. Description of the Related Art
It is difficult in the conventional printing method, which requires dampening water, to control the delicate balance between the dampening water and the ink. Consequently, the ink may become emulsified or mixed in with the dampening water causing an inferior ink density and background staining which leads to paper loss.
In contrast, waterless planographic printing plates have numerous advantages because they do not require dampening water. Various types of waterless planographic printing plates which allow planographic printing to be carried out without using dampening water are proposed in, for example: Japanese Patent Application Publication (JP-B) Nos. 44-23042, 46-16044, 54-26923, 56-14976, 56-23150, and 61-54222; and Japanese Patent Application Laid-Open (JP-A) Nos. 58-215411, 2-16561, and 2-236550.
Among the above waterless planographic printing plates are disclosed several having an extremely high performance. These waterless planographic printing plates comprise a substrate on which are disposed a primer layer, a photopolymeric light-to-heat conversion layer, and a silicone rubber layer in that order. Portions of the photopolymeric light-to-heat conversion layer exposed by an exposure light undergo polymeric hardening and the strength of the adhesion thereof with the silicone rubber layer is increased. During developing, only the unexposed portions of the silicone rubber layer are peeled away to receive the ink and thus form an image.
However, due to the rapid advances in recent years of output systems such as pre-press systems, image setters, and laser printers, methods for providing printing plates have been proposed which use new plate-making methods such as computer-to-plate and computer-to-cylinder methods in which print images are converted into digital data. Therefore, new types of printing materials are sought after for use in these printing systems and the development thereof is proceeding steadily.
However, the majority of these technologies relate to conventional planographic printing plates which print using dampening water, and currently there is almost nothing known about such technologies for waterless planographic printing plates.
The waterless planographic printing plate precursors disclosed in, for example, U.S. Pat. No. 5,353,705, International Publication (WO) No. 9401280, and Japanese Patent Application Laid-Open (JP-A) No. 9-131978 are examples of waterless planographic plates which can be formed by laser writing. However, because the removal of the ink repellant silicone rubber layer in these technologies relies on ablation of the light-to-heat conversion layer by laser irradiation, the linearity of fine lines and the circularity of dots is very poor and not satisfactory in a print image, therefore, an improvement has been widely looked for. Moreover, in the conventional technology, because the adhesion between the light-to-heat conversion layer and the silicone rubber layer, as well as the resistance to solvents of the light-to-heat conversion layer are both poor, during handling of the printing plates, when the surface of the plate is being washed with solvent, or during the printing process, problems occur as the printing plate tends to scratch easily enabling ink to be thickly deposited in the scratches and thus staining non-image areas. In order to prevent scratches, the addition of a silane coupling agent to the silicone rubber layer has been disclosed. However, adding the silane coupling agent is insufficient in increasing the adhesion between the light-to-heat conversion layer and the silicone rubber layer, and the improvement in scratch resistance is also minimal.
The aim of the present invention is therefore to provide a waterless planographic printing plate precursor which is laser writable and has excellent scratch resistance, solvent resistance, and image reproducibility.
As the result of intense research, the present inventors have discovered that the aim of the present invention can be achieved by superposing in the following order on a substrate a light-to-heat conversion layer containing a compound for converting laser light to heat, and a silicone rubber layer to form a waterless planographic printing plate precursor, wherein the light-to-heat conversion layer is formed by the coating and drying on the substrate of a coating solution containing a metallic chelate compound and a compound for converting laser light to heat, and preferably further containing a polymer having active hydrogen in the molecule.
Namely, the present invention is a waterless planographic printing plate precursor comprising a substrate on which are superposed a light-to-heat conversion layer containing a compound for converting laser light to heat, and a silicone rubber layer in that order, wherein the light-to-heat conversion layer contains a reaction product of a metallic chelate compound and/or a self-condensation product of a metallic chelate compound. The light-to-heat conversion layer further preferably contains a polymer having active hydrogen in the molecule.
Moreover, the present invention provides a method for producing a waterless planographic printing plate precursor, wherein a light-to-heat conversion layer is formed by coating on a substrate a first coating solution containing a compound for converting laser light to heat and a metallic chelate compound, coating on the light-to-heat conversion layer a second coating solution containing silicone, and heating the coated film to form a silicon rubber layer.
In the present invention, during the coating and drying of the light-to-heat conversion layer, the metallic chelate compound reacts with a compound containing active hydrogen. If the metallic chelate compound reacts with water, hydrolysis and condensation reactions progress; if the metallic chelate compound reacts with a polymer having active hydrogen in the molecule, a cross-linked structure is formed. As a result of this, the resistance to solvents of the light-to-heat conversion layer is improved due to the chemical bonding and the physical interlocking of the polymer molecules. Moreover, the adhesion between the light-to-heat conversion layer and the silicone layer when the silicone layer is being applied and dried is increased which improves the scratch resistance. The reason for the increase in the adhesion between the light-to-heat conversion layer and the silicone layer is as yet unclear, however, it is thought to be due to the formation of a covalent bond or to the formation of an extremely strong interaction between the residue of the reaction product of the metallic chelate compound contained in the light-to-heat conversion layer and the silicone cross-linking agent contained in the silicone layer.